The use of catalytic processes for combustion or oxidation is a well-known method for potentially reducing levels of nitrogen oxides (NOx) emissions from gas turbine engine systems. There are various processes for converting the chemical energy in a fuel to heat energy in the products of the conversion. The primary processes are: 1) gas phase combustion, 2) catalytic combustion, and 3) catalytic oxidation. There are also combinations of these processes, such as processes having a first stage of catalytic oxidation followed by a gas phase combustion process (often referred to as cata-thermal). In catalytic oxidation, an air-fuel mixture is oxidized in the presence of a catalyst. In all catalytic processes the catalyst allows the temperature at which oxidation takes place to be reduced relative to non-catalytic combustion temperatures. Lower oxidation temperature leads to reduced NOx production. In catalytic oxidation all reactions take place on the catalytic surface; there are no local high temperatures and therefore the lowest possible potential for NOx to be formed. In either catalytic combustion or catathermal combustion, some part of the reaction takes place in the gas phase, which increases local temperatures and leads to higher potential for NOx being formed. Using catalytic oxidation, NOx levels less than one part per million can be achieved under optimum catalytic oxidation conditions; such low levels in general cannot be achieved with conventional non-catalytic combustors, catalytic combustion, or cata-thermal combustion. In the present application, the term “catalytic combustor” is used to refer to any combustor utilizing catalysis, preferably one utilizing catalytic oxidation.
The catalyst employed in a catalytic combustor tends to operate best under certain temperature conditions. In particular, there is typically a minimum temperature below which a given catalyst will not function. For instance, palladium catalyst requires a combustor inlet temperature for the air-fuel mixture higher than 800 K when natural gas is the fuel. In addition, catalytic oxidation has the disadvantage that the physical reaction surface which must be supplied for complete oxidation of the hydrocarbon fuel increases exponentially with decreasing combustor inlet temperatures, which greatly increases the cost of the combustor and complicates the overall design. The need for a relatively high combustor inlet temperature is one of the chief reasons why catalytic combustion in general, and catalytic oxidation in particular, has not achieved widespread use in gas turbine engine systems. More specifically, such high combustor inlet temperatures generally cannot be achieved in gas turbines operating with compressor pressure ratios less than about 40 unless a recuperated cycle is employed. In a recuperated cycle, the air-fuel mixture is pre-heated, prior to combustion, by heat exchange with the turbine exhaust gases. Recuperation thus can help achieve the needed combustor inlet temperature for proper catalyst operation, at least under some conditions. However, there are often other operating conditions that will be encountered at which the minimum required combustor inlet temperature still cannot be achieved even with recuperation.
For instance, when recuperation is applied in small gas turbines, material temperature limitations in the recuperator can limit the maximum air or air-fuel mixture temperature. As an example, with conventional high-temperature materials in the recuperator, the maximum safe operating temperature of the recuperator may be about 900 K, and hence an air-fuel mixture temperature of about 800 to 850 K is about the highest that can be achieved. This temperature range is higher than the minimum catalyst operating temperature for some types of catalysts and therefore the catalytic combustor may operate properly at one particular operating condition such as 100 percent load and standard-day ambient conditions. At other operating conditions, however, such as part-load and/or cold ambient conditions, the combustor inlet temperature may fall below the minimum.
It would be desirable to be able to overcome such problems so that the low-NOx potential of catalytic oxidation could be realized in small gas turbine engine systems. Additionally, there are other benefits that can be achieved with catalytic processes. These processes extend the operating flammability limits of gaseous hydrocarbon fuels, including but not limited to landfill gases, anaerobic digester gases, natural gas, and methane. Thus, the process can take place at much more dilute (leaner) fuel/air ratios than conventional combustion. This allows the fuel gas to be mixed with the air prior to or during the compression process, resulting in a uniform fuel-air mixture entering the combustor. This in turn allows the elimination of a fuel gas compressor, which is very costly particularly for small gas turbines. Fuel gas compressors may add $60/kW or more to the cost of the engine, which is typically in the range of $600–$900/kW. Furthermore, the fuel gas compressor detracts from the reliability and availability of the engine, since it must operate in order for the engine to operate, and adds to the cost of maintenance because of oil, filters, mechanical or electrical wear out, and the like.